1. Field of the Invention
The present invention relates to a method of driving a cholesteric liquid crystal display (LCD) panel, and more particularly, to a method of driving a cholesteric LCD panel by applying at least first, second, and third voltages to cholesteric liquid crystal cells of the cholesteric LCD panel.
2. Description of the Related Art
Cholesteric LCD panels are reflective LCD panels having a structure in which cholesteric liquid crystal is filled among transparent electrode lines formed of, for example, indium-tin-oxide (ITO), which are arranged on two transparent substrates, for example, glass substrates, facing each other.
The fundamental characteristics of a cholesteric liquid crystal cell is shown below. When a voltage higher than a first threshold voltage is applied to a cholesteric liquid crystal cell, the cholesteric liquid crystal cell changes into a homeotropic state. In the homeotropic state, molecules of the cell are vertically arranged with respect to the surface of the cell.
When the voltage, which is lower than the first threshold voltage and is higher than a second threshold voltage, is applied to the cholesteric liquid crystal cell in the homeotropic state, specifically, when the voltage that is applied to the cell in the homeotropic state is gradually lowered, the cell changes from the homeotropic state into a focal conic state. In the focal conic state, the molecules of the cell are arranged in a helical structure, and a helical axis is nearly parallel to the surface of the cell. Accordingly, light is mostly transmitted without being reflected so that the cell is almost transparent.
When the voltage, lower than the second threshold voltage, is applied to the cholesteric liquid crystal cell in the homeotropic state, specifically, when the voltage that is applied to the cell in the homeotropic state is rapidly lowered, the cell changes from the homeotropic state via a transient planar state and incomplete-planar state into a planar state. In the planar state, the molecules of the cell have a periodic helical structure, and a helical axis is perpendicular to the surface of the cell. Accordingly, only light having a wavelength corresponding to the product nP of an average refractive index “n” of the cholesteric liquid crystal cell and a helical pitch P can be reflected. Meanwhile, the transient-planar state has a similar structure to the planar state and has about twice longer helical pitch than the planar state. The incomplete-planar state is a variable state appearing in the middle of relaxation from the transient-planar state into the planar state.
The focal conic state and the planar state have a memory effect through which the states are maintained for a long period of time even if supply of voltage is stopped. Due to such memory effect produced by bistability, the planar state and the focal conic state are employed depending on selection of a certain cholesteric liquid crystal cell in cholesteric LCD panels, thereby decreasing power consumption. In addition, since cholesteric LCD panels use a selective reflection driving scheme due to their characteristics, they have a high luminance characteristic.
Exemplars in the art include U.S. Pat. No. 5,748,277 issued to Huang et al. for “Dynamic Drive Method and Apparatus for a Bistable Liquid Crystal Display,” and U.S. Pat. No. 6,154,190 issued to Yang et al. for “Dynamic Drive Methods and Apparatus for a Bistable Liquid Crystal Display.
I have found that in the art, the internal circuit of the scan-electrode driving device is complicated, thereby increasing manufacturing costs.